Technical Field
The invention relates to the field of charged particle radiation therapy. More specifically, the invention relates to the field of spot scanned ion therapy. More specifically, the invention relates to a charged particle system for the irradiation of a target of tissues that may be cancerous. The invention also relates to a method for irradiation of a target with a particle pencil beam.
Related Art
In charged particle radiation therapy, a number of irradiation techniques are known today. The most common form of radiation therapy currently is photon therapy. However, photon therapy comes with several complications. For one, when using photon therapy, the applied photon beam passes through a targeted tumor and exits the patient through healthy tissue distal to the tumor. The exiting of the photon beam or dose through the healthy tissue increases the difficulty in preventing radiation damage to the healthy tissue. The radiation damage caused by the exiting dose through healthy tissue also is a limiting factor when designing an effective tumor treatment plan.
Ion therapy, which includes proton therapy and argon, carbon, helium and iron ion therapy, amongst others, provides some advantages over photon therapy. For one, ion therapy can result in a lower total radiation energy, termed integral dose, being deposited in a patient for a given tumor dose in relation to photon therapy. The integral dose reduction is significant because it reduces the probability of stochastic effects, i.e., patients developing secondary malignant neoplasms following irradiation of non-tumor tissue. Young patients with high probabilities of long term survival have a higher probability of developing secondary malignant neoplasms than older patients since the probability of development is related to the time elapsed post-therapy. Thus, the reduction of radiotherapy doses to non-tumor tissues in children is a particularly important advantage of ion therapy. The integral dose reduction for proton therapy relative to photon therapy has been quantified for parameningeal paraorbital rhabdomyosarcoma and spinal neuraxis in children with medulloblastoma, resulting in a reduction in the probability of radiation-induced secondary malignancies by factors of ≧2 and 8-15, respectively. Proton therapy is expected to reduce the probability of occurrence of secondary malignant neoplasms in adults as well. For example, the probability of a secondary malignant neoplasm is decreased by 26% to 39% for prostate patients receiving proton therapy versus intensity modulated photon therapy.
The second clinical advantage of ion therapy over photon therapy is that radiation dose to healthy tissues is reduced sufficiently such that deterministic effects (i.e., complications whose magnitude is related to the radiation dose delivered) may be reduced relative to photon therapy. Examples of deterministic effects are skin erythema and xerostomia. The reduction in deterministic effects has been demonstrated in multiple studies in which tumor dose conformity has been shown to be comparable to that of photon therapy, but healthy tissue sparing for proton therapy is superior. Healthy tissues associated with multiple tumor sites have been shown to be spared of more dose by proton than photon therapy, including paraspinal sarcomas, head-and-neck malignancies, meningioma, cervix, medulloblastoma, paranasal sinus, and prostate.
Spot scanning (SS), an advanced form of ion therapy delivery, has some advantages over traditional ion therapy. Conventional proton therapy beams for treating patients are typically generated using either passive scattering or uniform dynamic scanning. With passive scattering, one or more range compensators and a range modulator are used to spread a proton pencil beam into a beam that produces a spatially uniform dose distribution laterally and in depth. The range modulator may be a spinning propeller, wedge, or ridge filter, and produces a spread out Bragg peak (SOBP). The field is shaped laterally to the central beam axis with a custom-designed aperture, block, or multi-leaf collimator (MLC), and is shaped in depth to match the distal edge of the treatment volume using a patient-specific compensator. Single and double scattering systems exist, the latter typically providing larger regions of uniform dose than the former. Uniform dynamic scanning uses a magnetically scanned pencil beam and dynamic energy modulation to generate proton fields which, when averaged over time, have a uniform intensity in space. Field shapes are defined by apertures or blocks in a similar manner as with passive scattering.
In SS ion therapy, the treatments are delivered with pencil beams, usually produced by a beam generator (e.g., a cyclotron), that are magnetically scanned to deliver dose in the target. The size of the pencil beam in SS is generally much smaller than uniform dynamic scanning. The use of pencil beams allows the beam shape to be defined using the scanning magnets rather than an aperture. This pencil beam spot scanning technique represents an advance over the single or double scattering technique, wherein a scattered broad beam is shaped by a patient specific collimator or aperture, so that it corresponds to the shape of the target to be treated. As a result, the lateral falloff of dose distributions delivered with spot scanning without an aperture is dependent on the size of the incoming pencil beam and interactions of the beam in the patient.
Additionally, in SS, the beam intensity, when averaged over time, is not required to be uniform. This allows intensity modulated proton therapy (IMPT) to be delivered. With IMPT, several fields can be optimized simultaneously such that the sum of all fields will yield a uniform dose to the target while minimizing the dose to surrounding normal structures.
However, proton SS systems have low-energy (≦160 MeV) lateral beam intensity profiles that are less sharp than those of photon therapy systems, thus more of the radiation dose is typically deposited lateral to the tumor for low-energy treatments (i.e., the lateral penumbra of a pencil beam is larger than the penumbra of a collimated broad beam). As a result, proton SS is superior to photon therapy in integral dose delivered and inferior to conventional proton therapy in dose delivered lateral to the tumor for low-energy treatments. The degree of inferiority imposed by the latter property is dependent upon the energy of the ion beam, as low energy beams tend to be broader than higher energy beams due to the physical properties of the system used to transport the ion beam from the accelerator to the patient.
Therefore, attempts have been made to reduce the size of the penumbra. For example, a device to reduce the penumbra of a pencil beam spot scanning is disclosed in U.S. Patent Application Publication No. US 2013/0043408. However, the device consists of a patient specific collimator or aperture to be inserted in the beam line. A patient specific collimator means an individual collimator for each patient has to be constructed, adding to the overall cost of treatment. MLCs have been used with pencil beam spot scanning, but MLCs are complex to develop and require a lot of space such that MLCs are prevented from being positioned in a very close proximity to the patient. In addition, the weight of such an MLC requires a strong mechanical structure to support it.
Therefore, there is a need for a system and method for the application of SS ion therapy that reduces the radiation dose delivered to healthy tissues outside the target boundary. In addition, there is a need for a system that allows the application of SS ion therapy at areas of a patient in which access is difficult (e.g., areas around the neck and head due to the location of the patient's shoulders). There is also a need for a simplified and cost effective device for reducing the lateral penumbra of a beam from a SS system.